Fly ash is a byproduct of coal fired power plants. Much fly ash has a carbon content which precludes any beneficial use thereof. Nonbeneficiated high carbon content fly ash must be disposed of at certain designated sites after obtaining the necessary permits. Processes are known in the art, however, for beneficiating fly ash particles by oxidizing carbon therein so as to reduce the carbon content to acceptable levels. See, for example, U.S. Pat. No. 5,160,539 filed Apr. 5, 1991 which discloses a method for reducing the carbon content in fly ash particles so as to permit the particles to be useful as pozzolan and replace a portion of the cement in concrete. That process, however, is limited to bed temperatures in excess of about 1300.degree. F. and preferably between about 1500.degree. and 1600.degree. F.
There are several reasons for operating fluid beds in the 1500.degree. F. range, including maintaining a large temperature differential for heat transfer and optimizing sulfur capture. Operation in such a range also stabilizes combustion by providing a substantial margin between the autoignition temperature of the fuel and the bed temperature. The autoignition temperature is the temperature at which the oxidation of the fuel accelerates greatly and is commonly thought of as the temperature at which the fuel begins to burn. Such a substantial margin is particularly desirable for fuels having a low volatile content.
Certain types of fly ash can not be beneficiated in the 1300.degree. to 1500.degree. F. temperature range disclosed in U.S. Pat. No. 5,160,539 because undesirable sintered agglomerates begin to form from the fly ash. The temperature at which this begins, often referred to as the incipient agglomeration temperature, is always lower than the initial deformation temperature for that fly ash and must be determined empirically for each fly ash feedstock. As can be appreciated by those skilled in the art, the initial deformation temperature of a fly ash is the temperature at which a specified amount of the fly ash first begins to melt under specified laboratory conditions. Fly ash from certain coal types, often referred to as low fusion fly ash, have chemical characteristics which can cause partial melting of the fly ash at relatively low temperatures. The low initial deformation temperature of these fly ashes can be deleterious to the beneficiation of the fly ash. Should such partial melting occur in a fluid bed of fine ash particles, for instance, agglomeration of the particles may result. These defluidized particles or agglomerates fall to the bottom of the bed, disrupt the uniform fluidizing air flow and cause a high temperature defluidized zone. This, in turn, causes further agglomeration in an expanding cycle. Because of the foregoing, fly ash having an incipient agglomeration temperature less than 1300.degree. F. can not be processed by the method of U.S. Pat. No. 5,160,539.
In addition to the basic fusion characteristics of fly ash, the presence of alkali metals such as sodium can markedly reduce the incipient agglomeration temperature. For fly ash with low concentrations of alkali metals, the incipient agglomeration temperature in fluid beds is often 300.degree. to 600.degree. F. below the initial deformation temperature of the fly ash. Fly ash having an incipient agglomeration temperature of less than 1300.degree. F., whether due to low initial deformation temperature or alkali metal content or other factors, can not be processed by the method of U.S. Pat. No. 5,160,539.
Autoignition of low volatile, low heat content carbonaceous fuels would not be expected below 1300.degree. F. Fly ash has a very low volatile content and a very low heat content.
It has not previously been thought possible to operate a fly ash carbon burnout fluid bed combustor in a continuous mode at less than 1300.degree. F. Laboratory tests were run in June 1989 using a modification of the Loss On Ignition (LOI) test (ASTM C-311 and C-114) to bracket potential operating ranges for development of the fly ash carbon burnout process described in U.S. Pat. No. 5,160,539. In these tests, essentially all carbon was consumed from a small sample of fly ash (perhaps 0.2 gram) at a temperature of 1382.degree. F. in ambient air over a period of two hours. The tests showed that even with the extremely long residence time, and constant oxygen concentration, less than 60% of the carbon was consumed at 1000.degree. F. and less than 1% at 500.degree. F. Graphing this data gives a surprisingly linear relationship and seems to indicate that, even with excessively long residence times and high oxygen content, operation below 1300.degree. F. would reduce less than 90% of the carbon content. An actual fly ash carbon burnout fluid bed would have much shorter residence times, on the order of 30 minutes, and reduced oxygen availability, about 5% 0.sub.2 by volume in exhaust gas. Therefore, poorer carbon reductions would be expected at bed temperatures similar to those discussed above.
Relevant literature provides several indirect indications that autoignition of the low BTU, low volatile fly ash "fuel" should not occur below 1300.degree. F. For example, the 15th edition of the NFPA (National Fire Protection Association) Fire Protection Handbook on page 4-86 lists the following ignition temperatures in a turbulent cloud, which is similar to conditions in a fluid bed, for the low-volatile carbonaceous dusts specified below.
______________________________________ Lamp Black 1346.degree. F. Carbon Black, Acetylene No Ignition Carbon, Petroleum Coke and 1310.degree. F. Pitch Electrodes Coal, Pennsylvania Anthracite 1346.degree. F. Coke, Petroleum 1238.degree. F. Graphite No Ignition ______________________________________
Fly ash is similar to these materials except that the carbon content is far lower, being a high fraction of inert materials, and the volatile content is substantially lower than both typical petroleum coke and Pennsylvania anthracite. The NFPA handbook on pages 4-92 and 4-93 indicates that while the very small particle size of the fly ash may tend toward a reduction in ignition temperature, both admixture of inert material and reduced oxygen content, which is typical during operation of a fly ash carbon burnout process, tend to raise the ignition temperature.
These tendencies are also noted in "Development and Control of Dust Explosions" by Nagy and Verakis. On page 48 of this book, a graph of ignition temperature versus volatile content is provided for a number of materials, including carbonaceous dust. As the volatile content of the carbonaceous dust approaches zero, the average ignition temperature shown is about 1450.degree. F. This reference also includes a graph on Page 49 which shows the ignition temperature of mixtures of highly volatile Pittsburgh bituminous coal and inert Fuller's earth. While the ignition temperature of the coal alone is about 1100.degree. F., the ignition temperature of a mixture of 80% inert material and 20% Pittsburgh coal is approximately 1325.degree. F.
Some references, such as Catalytic Cracking Of Heavy Petroleum by Decroocq, note that catalytic cracker regenerators remove carbonaceous deposits from fine catalyst beads in fluid beds at temperatures in the 1100.degree.-1175.degree. F. range. However, other references, such as Petroleum Refinery Engineering by Nelson indicate that the carbonaceous deposits on the catalyst beads have a substantial volatile content which includes 7-15% hydrogen. As previously noted, these volatile hydrocarbons would be expected to very substantially lower the autoignition temperature of the material thus allowing combustion in the cited temperature range.
Fluid bed combustors designed to operate at temperatures less than 1300.degree. F., and particularly less than 1050.degree. F., require less exotic construction materials than combustors which operate at higher temperatures and, as a result, may be constructed more economically than high temperature combustors.